The present invention relates generally to apparatus for detecting ammonia in fluids and, more particularly, to the use of electronic absorption spectroscopy for determining the quantity of ammonia that has reacted with a conducting polymer film in contact therewith.
Conducting polymers which can be prepared by a simple oxidative polymerization method have found use as chemical and biological sensors. They exhibit reversible pH-induced spectroscopic and gas-induced conductivity changes. Such materials also provide a suitable structure for immobilization of ligands, enzymes, and antibodies. See, e.g., xe2x80x9cImmobilization Of Glucose-Oxidase In Ferrocene-Modified Pyrrole Polymersxe2x80x9d by N. C. Foulds and C. R. Lowe, Analytical Chemistry 60, 2473-2478 (1988); xe2x80x9cPulsed Amperometric Detection Of Proteins Using Antibody Containing Conducting Polymersxe2x80x9d by O. A. Sadik and G. G. Wallace, Analytica Chimica Acta 279, 209-212 (1993); and xe2x80x9cOptical Sensing Of pH Based On Polypyrrole Filmsxe2x80x9d by S. Demarcos and O. S. Wolfbeis, Analytica Chimica Acta 334, 149-153 (1996).
Conducting polymer gas sensors commonly rely on conductivity changes that occur when they are exposed to certain gases. For example, the dc conductivity of a polypyrrole film decreases with increasing ammonia gas concentration, and an ammonia gas sensor based on this property has been developed (See, e.g., xe2x80x9cThe Effect Of Initial Conductivity And Doping Anions On Gas Sensitivity Of Conducting Polypyrrole films to NH3 xe2x80x9d by M. Bile et al., Sensors and Actuators B37, 119-122 (1996)). At room temperature, the response time of such a sensor was found to be a few tens of minutes. By increasing the temperature from 20 to 100xc2x0 C., the response time was shortened by a factor of five. After treatment with NO2, the response and sensitivity of the sensor deteriorated. The major problems of this polypyrrole ammonia gas sensor are slow response time, low sensitivity, irreversible response, and a controlled high temperature (100xc2x0 C.) requirement.
The dc conductivity of polyaniline films also changes when the films are exposed to ammonia gas. For example, a polyaniline film containing nickel prepared by electrochemical oxidation can be used to detect ammonia gas in the range between 1 and 10,000 PPM at room temperature (See, e.g., xe2x80x9cPolymer Film-Based Sensors For Ammonia Detectionxe2x80x9d by S. A. Krutovertsev et al., Sensors And Actuators B7, 492-494 (1992)). The response time was reported to be approximately two minutes which is much faster than that for a polypyrrole ammonia sensor; however, the regeneration of the polyaniline sensor was slow (See, e.g., xe2x80x9cAmmonia Sensors Based On Sensitive Polyaniline Filmsxe2x80x9d by A. L. Kukla et al., Sensors and Actuators B37, 135-140 (1996)). By heating the sensor layer to between 104 and 107xc2x0 C., it was possible to completely regenerate the sensor within a short period of time. Polyaniline films are also sensitive to H2S, NOx, and SO2. Detection limits as low as 4 PPM can be achieved for H2S and NOx gases with polyaniline gas sensors (See, e.g., xe2x80x9cPolyaniline Thin Films For Gas Sensingxe2x80x9d by N. E. Agbor et al., Sensors and Actuators B28, 173-179 (1995)). In a variation of these measurements U.S. Pat. No. 5,252,292 for xe2x80x9cAmmonia Sensorxe2x80x9d which issued to Hirata et al. on Oct. 12, 1993 describes an ammonia sensor consisting of at least one pair of electrodes and an ammonia-sensing material comprising a polyaniline filling the space between the electrodes. Therein, the polyaniline changes its electric resistance in proportion to the ammonia concentration in an atmosphere such as air or other gas and accordingly the measurement of the electric resistance enables the detection of the ammonia concentration at a high sensitivity.
Recently, high-frequency and multi-frequency ac conductivity measurement techniques have been used for conducting polymer gas sensors (See, e.g., xe2x80x9cHigh-Frequency a.c. Investigation Of Conducting Polymer Gas Sensorsxe2x80x9d by F. Musio et al., Sensors and Actuators B23, 223-226 (1995) and xe2x80x9cMulti-Frequency Measurements Of Organic Conducting Polymers For Sensing Of Gases And Vaporsxe2x80x9d by M. E. H. Amrani et al., Sensors and Actuators B33, 137-141 (1996)).
The most important advantage of ac conductivity measurements is that it is possible to distinguish different chemical species with a single sensor. Organic vapors, such as methanol, acetone, and ethyl acetate, were detected by measuring a.c. conductivity changes of a polyaniline gas sensor at different frequencies (M. E. H. Amrani et al., supra). Another technique, which can differentiate different chemical species, is the frequency counting interrogation technique (See, e.g., xe2x80x9cFrequency Counting Interrogation Techniques Applied To Gas Sensor Arraysxe2x80x9d by M. E. H. Amrani et al., Sensors and Actuators B57, 75-82 (1999). In order to monitor characteristic resistance and capacitance changes simultaneously, a conducting polymer sensor was used as one of the arms of a four-channel Wien-bridge oscillator system. From the combined patterns of frequency changes in the four channels, it was possible to detect the vapor to which the system was exposed.
There are a few reports of conducting polymers being used for optical gas sensors (See, e.g., xe2x80x9cAn Optical Gas Sensor Based On Polyaniline Langmuir-Blodgett Filmsxe2x80x9d by N. E. Agbor et al., Sensors and Actuators B41, 137-141 (1997)). Therein, a polyaniline optical sensor based on the surface plasmon resonance and sensitive to NO2 and H2S with detection limits of approximately 50 vapor parts per million was described. However, the sensor response was slow, and total regeneration of the sensor after exposure to gases was impossible.
In U.S. Pat. No. 6,051,437 for xe2x80x9cOptical Chemical Sensor Based On Multilayer Self-Assembled Thin Film Sensors For Aquaculture Process Controlxe2x80x9d, which issued to Luo et al. on Apr. 18, 2000 describes optical chemical probes having layers of anionic and cationic polyelectrolytes and one or more dyes incorporated into these layers. The probes are placed in the medium to be analyzed and the dye or dyes react in the presence of the corresponding chemical. Color changes may be observe manually or by a photodetector. A light source may be employed to increase the optical signal received from the probe.
In U.S. Pat. No. 6,117,686 for xe2x80x9cMethod For Detecting Harmful Gases Which Is Applicable To Broad Gas Concentration Rangexe2x80x9d, which issued to Tanaka et al. on Sep. 12, 2000, a method for detecting certain gases is described wherein a layer of matrix polymer which includes tetraphenylporphyrin (TPP) is used as a detector. When the concentration of tetraphenylporphyrin contained in the matrix polymer is increased, an absorption peak appears at approximately 718 nm; moreover, when the concentration of tetraphenylporphyrin is altered, the gas concentration at which the absorption peak appears also changes, as measured by transmittance or reflection of light from the detector. For example, the 718 nm feature begins to appear at a higher gas concentration when a detector containing a lower concentration of tetraphenylporphyrin is used, whereas the feature appears at a lower gas concentration when a detector containing a higher concentration of the pigment is employed. Tanaka et al. states that this phenomenon is observed specifically with tetraphenylporphyrin, and that it is possible to detect and quantify certain gases over a broader range of concentration using a plurality of detectors containing tetraphenylporphyrin in different concentrations in a matrix polymer.
Accordingly, it is an object of the present invention to provide an ammonia gas sensor based on polyaniline which is suitable for analytical chemistry use.
Another object of the invention is to provide an ammonia gas sensor based on polyaniline having rapid response time and capable of rapid regeneration.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the apparatus for quantitatively determining the amount of ammonia in a fluid hereof includes: a transparent polyaniline film; means for exposing the film to a fluid containing ammonia such that ammonia is absorbed onto the polyaniline film; means for directing light having a chosen wavelength through the film; means for detecting the light passing through the film; and means for comparing the amount of light passing through the film with the amount of light detected when there is not ammonia present in the fluid, whereby the amount of ammonia in the fluid is quantitatively determined.
Benefits and advantages of the invention include a robust portable, quantitative, sensitive ammonia detector having rapid response time and rapid regeneration capability.